What is a Probability Distribution?

A probability distribution is a table or an equation
that links each possible value that a
random variable can assume
with its probability of occurrence.

Discrete Probability Distributions

The probability distribution of a
discrete
random variable can always be represented by a table.
For example, suppose you flip a coin two times. This simple
exercise can have four possible outcomes: HH, HT, TH, and TT.
Now, let the variable X represent the number of heads that result from the
coin flips. The variable X can take on the values 0, 1, or 2; and
X is a discrete random variable.

The table below shows the probabilities associated with each
possible value of X. The probability of getting 0 heads
is 0.25; 1 head, 0.50; and 2 heads, 0.25. Thus, the table is
an example of a probability distribution for a discrete random variable.

Number of heads, x

Probability, P(x)

0

0.25

1

0.50

2

0.25

Note: Given a probability distribution, you can find
cumulative probabilities. For example,
the probability of getting 1 or fewer heads [ P(X < 1) ] is
P(X = 0) + P(X = 1), which is equal to 0.25 + 0.50 or 0.75.

Continuous Probability Distributions

The probability distribution of a
continuous
random variable is represented by an equation,
called the probability density function (pdf).
All probability density functions satisfy the following
conditions:

The random variable Y is a function of X; that is, y = f(x).

The value of y is greater than or equal to zero for all
values of x.

The total area under the curve of the function is equal to one.

The charts below show two continuous probability distributions.
The chart on the left shows a probability density function
described by the equation y = 1 over the range of 0 to 1 and y = 0
elsewhere.
The chart on the right shows a probability density function
described by the equation y = 1 - 0.5x over the range of 0 to 2 and y = 0
elsewhere. The area under the curve is equal to 1 for
both charts.

y = 1

y = 1 - 0.5x

The probability that a continuous random variable falls in the
interval between a and b is equal to the
area under the pdf curve between a and b.
For example, in the first chart above, the shaded area shows
the probability that the random variable X will
fall between 0.6 and 1.0. That probability is 0.40.
And in the second chart, the shaded area shows
the probability of falling between 1.0 and 2.0.
That probability is 0.25.

Note: With a continuous distribution, there are an infinite
number of values between any two data points. As a result,
the probability that a continuous random variable will assume a
particular value is always zero. For example, in both of the above
charts, the probability that variable X will equal exactly
0.4 is zero.

Test Your Understanding

Problem 1

The number of adults living in homes on a randomly selected
city block is described by the following probability
distribution.

Number of adults, x

1

2

3

4 or more

Probability, P(x)

0.25

0.50

0.15

???

What is the probability that 4 or more adults reside at a
randomly selected home?

(A) 0.10
(B) 0.15
(C) 0.25
(D) 0.50
(E) 0.90

Solution

The correct answer is A. The sum of all the probabilities is equal
to 1. Therefore, the probability that four or more adults
reside in a home is equal to 1 - (0.25 + 0.50 + 0.15) or 0.10.